Technologies using semi-conducting materials such as for example:                hydrogenated amorphous silicon (aSiH), on insulating glass supports,        CMOS on crystalline silicon substrate,        charge-coupled circuits (CCD) on crystalline silicon substrate,make it possible to produce matrices of photosensitive points that are able to produce an image on the basis of visible or near-visible radiation. These matrices can nonetheless be used within the framework of the detection of radiological images. Accordingly, it suffices to interpose a scintillator screen between the X-ray radiation and the matrix so as to convert the X-ray radiation into luminous radiation in the band of wavelengths to which the photosensitive points are sensitive. The X-rays can also be converted directly into electrical charge by virtue of a photoconducting material (e.g.: a-Se, CdTe, PbO, PbI2, etc.). In this case, the photosensitive points each comprise a charge storage capacitor.        
The photosensitive points which form these matrices generally comprise a photosensitive element associated with an element fulfilling an on/off switch function. The photosensitive point is mounted between a row conductor and a column conductor. According to requirements, the photosensitive device then comprises a plurality of photosensitive points disposed as a matrix or strip.
The photosensitive element commonly consists of a diode, mounted in series with the on/off switch element. The on/off switch element may be for example a so-called switching diode whose “passing” or “on” state corresponds to the bias which turns it on in the forward direction, and whose “blocked” or “off” state corresponds to the reverse bias thereof. The two diodes are mounted with opposite directions of conduction, in a so-called “head-to-tail” configuration. Such an arrangement is well known, notably from French patent application 86 14058 (Publication No. 2 605 166) in which are described a matrix of photosensitive points of the type with two diodes in “head-to-tail” configuration, a method for reading the photosensitive points and a way of producing a photosensitive device such as this.
The column conductors are linked to a reading circuit converting the charge accumulated in the photosensitive elements into signals and delivering these signals as output. More particularly, the reading circuit can comprise several stages. Among these stages, a first stage can convert the charge into analog signals; a second stage can amplify these signals; a third stage can multiplex the signals so as to deliver “in series” and row after row as output from the reading circuit, a multiplexed signal comprising the signals representative of the charge accumulated in the photosensitive elements; and a fourth stage can convert the analog multiplexed signal into a digital signal. Like any electronic device, the reading circuit introduces electronic noise that gets added to the signals representative of the accumulated charge and limits the signal-to-noise ratio. In this instance, noise is generated at each stage of the reading circuit, and possibly further on in the image acquisition chain. More generally, noise is introduced at each modification or transformation of an analog signal, until this signal is converted into a digital signal. The noise is particularly troublesome insofar as the signals arising from the first stage may have a very small amplitude for certain modes of image acquisition, in particular when the X-ray radiation exposure time is small. The signal-to-noise ratio is then very small, for example close to unity, and the so-called useful signal, that is to say that representative of the charge accumulated in the photosensitive elements, may be difficult to extract from the overall signal, that is to say from the signal comprising at one and the same time the useful signal and the noise.
The generation of noise by the image acquisition chain may be limited by the design of the photosensitive device. However, noise generation depends greatly on the mode of use of the photosensitive device, notably the time for which the photosensitive points are exposed, the speed of reading the charge and the amplification gain applied to the signals. Thus, during the design of the photosensitive device, a compromise must be made between the various possible modes of use.
A solution for decreasing the noise generated during the transfer of the charge accumulated in the various photosensitive points to a charge amplifier is known from the French patent application published under the number FR 2 625 593. This patent describes a method for reading passive photosensitive points where the following is performed N times successively: a transfer of the charge present on each column of the matrix to intermediate storage zones, a duplication of this charge, a column-wise averaging of the duplicated charge and a restoral to the columns of the original charge. The charge averages are thereafter transferred to the following stage. The noise introduced during each transfer to the intermediate storage zones is generally decorrelated from the other noise. The averaging therefore makes it possible to partially cancel said noise. However, though the solution described in this patent does indeed make it possible to globally reduce the noise introduced by the image acquisition chain, it is solely at the level of the transfer of charge between the photosensitive points and the charge amplifier. The noise introduced at all the other stages of the image acquisition chain is not reduced. Moreover, the solution described in this patent applies only to the reading of passive photosensitive points, that is to say photosensitive points where the charge is converted into voltages in the reading circuit and not directly in the photosensitive points, as is the case for active photosensitive points.